When a high-energy charged particle, such as an electron or ion beam strikes a sample, photons can be emitted depending on the sample material. This phenomena is known as cathodoluminescence (CL). The collection and detection of these photons in the wavelength range from ultraviolet (UV) through visible to infrared (IR), can provide a wealth of information about the sample under investigation. CL is typically examined with the sample in an electron microscope and collected by directing the photons to, for example a light sensor, image array or spectroscopy equipment, any or all of which may be located outside the electron microscope beam column. The inside of the electron microscope beam column is kept at low pressures so the electrons can travel to the sample without significant scattering by gas in the beam column. After the light is collected, it can be transmitted from the low-pressure environment, through an optical window, and into instruments that analyze the CL light.
A common way to collect photons emitted via CL is via a collection-mirror, which may be a paraboloid mirror, located on axis with the electron beam (e-beam) and either above (more typically) or below the sample or both above and below the sample. Where the collection-mirror is located above the sample, on axis with the e-beam, the mirror will have a hole to allow the e-beam to pass through the mirror to the sample unobstructed.
The CL signal, comprised of the emitted photons, contains much information about the sample which emits it. Analysis of the CL signal may make use of the total CL intensity, spectral information, polarization information, and angular resolved light emission. The CL signal is often weak and it is often important to preserve as much of the signal as possible for analysis. Furthermore, it is often important to have separate analysis optics and detectors, herein referred to as CL instruments that are each optimized for a specific signal. For example, if spectral information is not important for an analysis, it may be best to directly couple the collected light into a light sensor, rather than pass the light through a spectrometer, where some fraction of the light is lost, and then into a light sensor.
Aligning a CL mirror over a sample so that the focal point of the CL mirror is precisely at the spot on the sample where the electron beam strikes and the CL is emitted can be a difficult and time-consuming process. Because of this, performing more than one type of analysis on the CL light can be difficult if multiple instruments are installed on the microscope column and a CL mirror is aligned for each instrument. Multiple instruments for CL analysis may be combined, allowing the user to use the same CL collection-mirror without repositioning with respect to the sample for a plurality of measurements. One difficulty, however, with combined multiple instruments is directing the collected CL light to the different instruments with minimal loss of any information contained in the CL light, including, for example CL intensity, spectral information, polarization information, and angular resolved light emission.